1. Field of the Invention
The present invention relates to an offset antenna system which provides improved symmetry in the radiation pattern over prior art antennas and, more particularly, to an offset antenna system comprising a curved focusing main reflector, at least two conic subreflectors and a feedhorn where the combination of elements are oriented such that the feedhorn is disposed at the focal point of the combined reflectors in a manner to coincide with the equivalent axis of the antenna system.
2. Description of the Prior Art
The use of orthogonal polarizations is often required in radio systems to double the transmission capacity between two remote points. Orthogonal polarizations have been radiated by a circular corrugated wall feedhorn which produces a spherical wave having circular symmetry. By placing the feedhorn at the focus of a parabolic reflector, an antenna with circular symmetry in the far-field is obtained provided the paraboloid is centered around the feed axis. In such a configuration the feedhorn partially blocks the reflected wave. To avoid such partial blockage, the feedhorn axis has been offset which unfortunately has been found to cause asymmetry in the radiated pattern after reflection, resulting in undesirable cross-polarization components in the reflected waves. The same behavior occurs if, instead of a parabola, an arbitrary reflector system with a single axis of revolution is used. Generally, it has been found that the asymmetry of the reflected wave increases with the angle of incidence of the ray corresponding to the feedhorn axis.
Various arrangements have been disclosed for improving discrimination between two polarizations transmitted by an offset antenna. One such arrangement is disclosed in U.S. Pat. No. 4,024,543 issued to V. J. Vokurka on May 17, 1977 which relates to a parabolic antenna comprising a number of parabolic cylinder surfaces as reflectors mounted confocally with a common plane of symmetry, and a feedhorn whose plane is substantially perpendicular to the planes of symmetry of the reflectors next to the radiator in the path of the rays. The Vokurka antenna provides a low cross-polarization value by including more than two substantially parabolic surfaces with each pair of surfaces having in common one line focus and one plane of symmetry, the line-foci intersecting or crossing each other.
Although a reflection from an offset surface causes some asymmetry, it is known to combine two reflections with nonzero angles of incidence so as to insure substantially improved symmetry after two reflections. In this regard see, for example, the article "Elimination of Cross Polarization in Offset Dual-Reflector Antennas" by H. Tanaka et al in Electronics and Communication in Japan, Vol. 58-B, No. 12, 1975 at pp. 71-78 which relates to the optogeometrical condition for effective cancellation of the cross polarization in an offset dual-reflector antenna comprising a paraboloidal main reflector, a subreflector having a shape which is a quadratic surface of resolution and a feedhorn. Cross polarization cancellation is effected dependent on the types, whether concave or convex, and the eccentricity of the subreflector, and the angles of the axes of the main reflector, subreflector and feedhorn. Additionally, see for instance, U.S. Pat. No. 3,792,480 issued to R. G. Graham on Feb. 12, 1974 which discloses an antenna system comprising a feedhorn, a subreflector and a main reflector where the feedhorn is displaced from the axis of the main reflector, and the axis of the subreflector is transverse to the axis of the main reflector to reduce certain asymmetrics.
Although the prior art arrangements provide substantially improved cross-polarization discrimination, the problem remaining is to provide a reflector antenna system comprising three or more reflectors with perfect symmetry in the radiation pattern where perfect symmetry implies perfect performance in cross-polarization discrimination.